The invention relates to a rotorcraft rotor, particularly a helicopter main rotor, comprising a rotor head at which a hub intended to be driven in rotation about an axis of the rotor is connected to at least two rotor blades, the rotor also comprising a rotor head vibration suppressor also known as a rotor head resonator, that is to say a device for attenuating the vibrations generated by the movement of the rotationally driven rotor, the vibration suppressor being used at the rotor head to effectively counter the vibrations directly at the source where they are produced.
The desired effect is to improve the vibration comfort in the cabin of a helicopter by reducing the level of vibration in the cabin as far as possible using a rotor head vibration suppressor the weight and cost of which are as low as possible, the structure of which is as simple as possible, and which is economical to maintain, while at the same time being highly effective.
The level of vibration in the cabin of a helicopter depends chiefly on the dynamic excitation of the main rotor and on the dynamic response of the helicopter structure. The dynamic excitation of the rotor results from the aerodynamic loadings on the rotor blades and can be broken down into, on the one hand, a force in the plane of the rotor, or coplanar force, exerted in a plane perpendicular to the axis of rotation of the rotor, and, on the other hand, into a force said to be xe2x80x9cverticalxe2x80x9d because it is exerted along the axis of the rotor, and into a moment said to be xe2x80x9ccoplanarxe2x80x9d, in a plane perpendicular to the axis of the rotor.
It is known that the frequencies of the xe2x80x9cverticalxe2x80x9d forces along the axis of the rotor in a rotating frame of reference are multiples of bxcexa9 where b is the number of rotor blades and xcexa9 is the angular speed of rotation of the rotor and that the frequencies of the coplanar moments applied to the rotor are (nbxe2x88x921)xcexa9 or (nb+1 )xcexa9, where n is an integer at least equal to 1.
For these reasons, known rotor head vibration suppressors, designed to reduce the dynamic excitation of the rotor, are essentially of two types, the first type relating to coplanar vibration suppressors for countering the forces in a plane perpendicular to the axis of the rotor, and the second type relating to xe2x80x9cverticalxe2x80x9d or xe2x80x9cflapxe2x80x9d vibration suppressors for attenuating forces and vibrations parallel to the axis of the rotor and generated by the flapping of the blades, and to counter the coplanar moments.
FR-A-2 733 483 and U.S. Pat. No. 5 639 214 have recently proposed that the well known coplanar vibration suppressors with at least one mass urged by at least one spring, or of the type known as xe2x80x9cbifilarxe2x80x9d, be replaced by pendular coplanar vibration suppressors comprising at least one pendular body mounted to oscillate on a support coaxial with the rotor mast and driven in rotation about the axis of the rotor, directly by the rotor head, and so that the pendular body can oscillate about an axis of pendular movement which is spaced away both from the center of inertia of the pendular body and from the axis of the rotor to which the axis of pendular movement is substantially parallel.
The pendular bodies of such pendular coplanar vibration suppressors therefore move in a plane parallel to the plane of the rotor, or even coincident with it, and their movements generate coplanar inertial forces which oppose the forces exerted in the plane of the rotor.
The xe2x80x9cverticalxe2x80x9d or xe2x80x9cflapxe2x80x9d vibration suppressors most commonly used are also pendular vibration suppressors comprising at least one pendulum for each rotor blade, that is to say at least two pendulums per rotor and each pendulum comprises at least one pendular body connected by at least one rigid arm to a support driven in rotation about the axis of the rotor and on which the pendulum is mounted to oscillate via a pivoting connection about an axis of pendular movement spaced away from the center of inertia of the pendulum and from the axis of the rotor. However, in these known xe2x80x9cverticalxe2x80x9d pendular vibration suppressors, the support is mounted transversely on or in the root of the corresponding blade or the connecting member, generally known as the cuff, connecting this blade to the hub, the axis of pendular movement then being substantially perpendicular to the longitudinal axis or pitch-change axis of the corresponding blade, which is an axis that is substantially radial with respect to the axis of the rotor, to which the axis of pendular movement may also be substantially perpendicular when the pitch of the corresponding blade is neutral.
Pendular vibration suppressors of the coplanar or vertical type have the advantage of being more lightweight than vibration suppressors of the other known types, because pendular vibration suppressors use centrifugal force as the force that returns the pendular bodies, which allows them to adapt to suit the variations in rotor rotational speed, and therefore makes them self-adapting or self-adjusting, if the influence of the speed of travel of the helicopter is neglected.
Furthermore, pendular xe2x80x9cverticalxe2x80x9d vibration suppressors, which oppose the vertical forces of the rotor and the coplanar moments, have the advantage that the pendulums mounted on the blades or on the flapping masses consisting of the blades with their cuffs that connect them to the hub, pivot about axes of pendular movement which are relatively distant from the center of the hub, which makes it possible to increase the maximum tuning frequency and the effectiveness of the pendulums.
These embodiments, known in particular from U.S. Pat. No. 4,239,456 and GB-2 123 371, do, however, have two major drawbacks. First of all, the blade pendulums are difficult to adjust because they are subjected to the changing pitch of the blades. Secondly, the presence of the pendulums on the blades increases the stresses along the blades, both in terms of drag and in terms of flapping, and for various rotor harmonics.
Hence, the blade pendulums affect the dynamic response of the blades and, more specifically, the stresses in the blades increase at the frequency to which the vibration suppressor is tuned, in particular, and at the harmonics of the rotor speed, in general.
The object of the invention is to overcome these drawbacks of the known blade, or more generally, flapping mass, pendular vibration suppressors and to propose a xe2x80x9cverticalxe2x80x9d and pendular rotor head vibration suppressor, the operation and adjustment of which are not hampered by the pitch of the blades, which does not affect the dynamic response of the blades, does not introduce additional stress along the blades, and remains of a simple and economical structure, easy to maintain and of a limited mass while at the same time being highly effective, and which can be incorporated compactly into the rotor head.
To this end, the pendular xe2x80x9cverticalxe2x80x9d vibration suppressor equipping the rotorcraft rotor of the type described hereinabove is a vibration suppressor which comprises at least two pendulums, each of which comprises at least one pendular body connected by at least one rigid arm to said hub so that each pendulum rotates as one with said hub about the axis of the rotor, and mounted so as to oscillate with respect to said hub via a pivoting connection about an axis of pendular movement which is spaced from the center of inertia of the pendulum and from the axis of the rotor and is substantially perpendicular to a radial plane passing through the axis of the rotor and through the center of inertia of the corresponding pendulum without the possibility of the pendulum oscillating about an axis parallel to the axis of the rotor and concurrent with the axis of pendular movement so as to attenuate vibrations generated by the movement of the rotor, and is one wherein the mass and the inertia of each pendulum of a first group of at least two pendulums, and its lever arm between its center of inertia and the axis of pendular movement, and the radius of attachment between the axis of the pendular movement and the axis of the rotor, are chosen so that the natural frequency of the pendulum is tuned to attenuate coplanar moments, to (nb+1)xcexa9 or (nbxe2x88x921)xcexa9 where n is an integer at least equal to 1, b is the number of rotor blades and xcexa9 is the rotational speed of the rotor.
By using simple xe2x80x9cverticalxe2x80x9d pendulums, oscillating in radial planes (passing through the axis of the rotor) and fixed to the rotor hub either directly or via at least one corresponding support that rotates as one with the hub, the rotor blades are not loaded by these pendulums, which are not subjected to the variations in blade pitch, and of which the pendular movements, preferably in the gaps between the adjacent blades, without interfering with the blades, their members for connection to the hub, the means for retaining and articulating these connecting members to the hub, nor with the pitch levers or drag dampers generally located laterally on each side of the connecting members, generate xe2x80x9cverticalxe2x80x9d inertial forces in the radial planes of travel of the pendulums, which counter the coplanar moments depending on the frequency tuning applied.
Advantageously, the rotor head vibration suppressor according to the invention comprises hub xe2x80x9cverticalxe2x80x9d pendulums from the first group of pendulums, the natural frequency of which is adjusted to (bxe2x88x921)xcexa9, which corresponds to the greatest excitation affecting coplanar moments. This characteristic makes it possible to reduce the angular acceleration of the rotor head in (bxe2x88x921)xcexa9.
If the harmonic (b+1 )xcexa9 of the coplanar moments has also to be reduced, other xe2x80x9cverticalxe2x80x9d pendulums from the first group of pendulums of the vibration suppressor have their natural frequency adjusted to this harmonic.
Advantageously, the rotor head vibration suppressor with xe2x80x9cverticalxe2x80x9d pendulums oscillating on the hub and rotating with it additionally comprises pendulums from a second group of at least two pendulums, the natural frequency of which is tuned, to filter out xe2x80x9cverticalxe2x80x9d forces along the axis of the rotor, to nbxcexa9, particularly to the harmonic bxcexa9 of the vibrational speed of the rotor, because this harmonic corresponds to the greatest excitation. This natural frequency of the pendulums of the second group is also obtained by choosing the mass and the inertia of each pendulum, its lever arm between its center of inertia and the axis of pendular movement, and its radius of attachment between the axis of pendular movement and the axis of the rotor. This characteristic makes it possible to reduce the xe2x80x9cverticalxe2x80x9d (along the axis of the rotor) acceleration on the rotor head in bxcexa9.
To this end, it may be advantageous for the pendular vibration suppressor to comprise at least one double pendulum comprising two pendular bodies oscillating about one and the same axis of pendular movement and each connected to the hub or to the corresponding support by at least one respective arm, the two pendular bodies and their respective arms having two different natural frequencies.
These two different natural frequencies may be different enough to allow distinct harmonics of the vibrational state of the rotor to be countered, in which case these two frequencies are chosen from the frequencies (nbxe2x88x921)xcexa9, (nb+1)xcexa9 and nbxcexa9, but may also differ from one another only slightly so as to allow just one harmonic such as (nbxe2x88x921)xcexa9 or (nb+1)xcexa9 or nbxcexa9 to be attenuated, but at different speeds of travel of the helicopter.
In order to ensure that such a double pendulum is appropriately compact and can be integrated suitably into the rotor head, a first pendular body of the double pendulum may be connected to the hub or to the corresponding support by a first arm mounted to oscillate between two second arms which may, similarly, connect a second pendular body of the double pendulum to the hub or to the support so that the first pendular body and the first arm are imbricated between the two second arms and the second pendular body, and possibly the support.
In practical terms, the pendular vibration suppressor may comprise at least one support which rotates as one with said hub about the axis of the rotor and on which pendulums are mounted to oscillate by corresponding pivoting connections, said support being substantially star-shaped, having branches supporting pendulums and being fixed coaxially to the hub so that each branch of the star-shaped support extends substantially radially outward with respect to the axis of the rotor and supports an oscillating pendulum at its outer radial end.
However, it is also possible for the pendular vibration suppressor to comprise supports which rotate as one with the hub about the axis of the rotor, each support being substantially elongate and oriented substantially radially with respect to the axis of the rotor, each support supporting at least one oscillating pendulum at its outer radial end and being fixed by its inner radial end directly to the hub, preferably between two adjacent blades and respective members for connecting said blades to the hub.
Advantageously, for appropriate integration into the rotor head, each pendulum is mounted to oscillate substantially between two adjacent blades of the rotor and possibly respective members for connecting said blades to the hub, and, in addition, the pendular vibration suppressor may comprise as many pendulums as the rotor has blades.
In this case, and when the rotor hub is arranged as a substantially radial plate, in an advantageously simple structure, each support has its inner radial end shaped as a clevis which straddles the outer radial edge of the substantially radial plate-like hub between the members connecting two adjacent blades to the hub, and this end is fixed to the hub by bolted connections of axis substantially parallel to the axis of the rotor, and the outer radial end of each support is also shaped as a clevis, in which is fixed a bearing block for the pivoting connection of the corresponding pendulum about the corresponding axis of pendular movement.
According to an effective and economical embodiment which is simpler to maintain and allows excellent integration into the rotor head, at least one support, but preferably each of them, is advantageously a fitting with two substantially flat and elongate mounting plates kept spaced apart substantially one above the other by the edge of the hub, the bearing block and a ball-end articulating to the support one end of a drag damper, the other end of which is articulated to one of the two connecting members between which said support is fixed to the hub.
However, whether the support is common to all the pendulums of the vibration suppressor, to just some of them, or specific to each of the pendulums, or if there is no support, the pivoting connection of at least one pendulum to the corresponding support or to the hub may advantageously be a connection with at least one rolling bearing housed in a bearing block fixed to the support or to the hub and through which there passes a pivot pin which rotates as one with each arm of said pendulum around said axis of pendular movement.
In order to allow fine adjustment of the frequency of the pendular vibration suppressor, it is furthermore advantageous that, for at least one pendulum, and preferably each of them, at least one corresponding pendular body comprises at least one part equipped with fixing and positioning means collaborating with complementary means of each corresponding pendulum arm, to allow said part of the pendular body to be fixed to said arm in an adjustable position so that the natural frequency of said pendulum can be adjusted.
To this end, in a simple and practical embodiment, the adjustable pendular body may advantageously comprise two lateral parts fixed removably by a screw to a rigid central part extended by an arm for connection to said pivoting connection on the support, each lateral part having an exterior profile that is off centered with respect to the fixing screw and having a peg projecting toward the central part and lodged in a selected one of a number of holes made in the central part facing said lateral part so that the lever arm and thus the natural frequency of the pendulum can be adjusted.